The only consideration for LVM mirroring is that the disks are all in one volume group.
Beyond that, there is no other consideration other than a proper number of free partitions, regardless of the number of physical (or logical) disks.
Each logical volume (LV I.E. filesystem) can have a separate mirror (or two mirrors!).

Normal strictness rules require than for a given PP of that LV have its mirror on a different physical device.
man mklv
(snip..)
-s strict
Determines the strict allocation policy. Copies of a logical
partition can be allocated to share or not to share the same
physical volume. The strict parameter is represented by one of the
following:
y
Sets a strict allocation policy, so copies for a logical
partition cannot share the same physical volume. This is
the default for allocation policy.
n
Does not set a strict allocation policy, so copies for a
logical partition can share the same physical volume.
s
Sets a super strict allocation policy, so that the
partitions allocated for one mirror cannot share a physical
volume with the partitions from another mirror.

For example, assume you have four 200 GB disks (total of 800 GB), with some number of filesystems on each, or perhaps spanning one or more disks.
You could add two 500 GB disks, and mirror the LV's such that two disks worth of LV, mirror to a single disk.
Or if you had added a single 1000 GB disk, you could mirror all four disks to just one disk.'
You could add eight 100 GB disks to that original 4x200 GB disk set, and have each single mirror use to two disks for one LV mirror.

One such configuration for us is to mirror along disk boundaries. We lease our hardware so when we replace a storage array, I can remove a mirror copy from a subset of luns and remove those luns. When I add the new array, I add those luns into the volume group and create an LV copy on those new luns. Afterwards, I can replace or remove the remaining disk array in the same manor.
extendvg vg1 hdiskx [hdisky]
mklvcopy fs2_lv 2 hdiskx [hdisky]
syncvg -l fs2_lv
rmlvcopy fs2_lv 1 hdiska [hdiskb] # removes the copy from these luns.
reducevg vg1 hdiska [hdiskb]
this will force the copy to the destination hdisk leaving a clear split of hdisks assigned to the primary copy and the second copy assigned to the new hdisks. later I can drop the original copy and remove the older LUN's.

Also, using these commands in different sequence, you can replace a failed disk drive without loosing your data.

If mirroring across LUN boundaries doesn't matter to you and you want to mirror any to any, then the following example is fine in your case.

Show the current state of the Logical Volumes
lsvg -l vg1
vg1:
LV NAME TYPE LPs PPs PVs LV STATE MOUNT POINT
fs2_lv jfs2 10 10 2 open/syncd /opt/fs2

Make a second copy where we don't care where the copy is located. The basic rule is the second or third copy of an original PP cannot reside on the same LUN.
mklvcopy fs2_lv 2
lsvg -l vg1
vg1:
LV NAME TYPE LPs PPs PVs LV STATE MOUNT POINT
fs2_lv jfs2 10 20 2 open/stale /opt/fs2 <-- notice the stale

After the copy is created, the data must be synced up. You can see this in the STATE above that the LV is Stale.
syncvg -l fs2_lv

Show the LV again to check on it's STATE and the PP/LP ratio to make sure the copy was done.
lsvg -l vg1
vg1:
LV NAME TYPE LPs PPs PVs LV STATE MOUNT POINT
fs2_lv jfs2 10 20 2 open/syncd /opt/fs2
# Notice above the LP =10 and PP
. This is showing that it takes 2 Physical Partitions for each Logical Partition. The next command shows this in detail. You can see that the LV is splattered across two hdisks but for each LP there are two copies on different LUN's.

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